The correct answer is C) join two hawsers. A carrick bend is a type of knot used to join two ropes or lines together, particularly thick, heavy ropes known as hawsers. The carrick bend is a strong and reliable knot that can withstand significant tension and weight, making it well-suited for connecting two large diameter lines. The other answer choices are incorrect because: A) a stopper knot is used to prevent a line from running through a block or fairlead, not to join lines; B) joining lines of different sizes is better accomplished with a sheet bend or similar knot; and D) adding strength to a weak spot in a line is typically done with a splice, not a knot.